The standard metadata facilities of the pg client should be accessible through Teiid as we emulate the pg system tables - pg_class, pg_attribute, etc. and have special handling for a couple of the expected metadata queries from the driver. What end client are you using, what version of the pg client are you using, and do you see any errors in the server log?
This really depends upon the client program you are using to access the metadata. If you are writing a C/C++ program, this link provides the ODBC API you can use http://msdn.microsoft.com/en-us/library/ms714562%28v=vs.85%29.aspx BTW, there is no single query to make this happen, it is a combination multiple API calls.